Aluminum alloy sheet having good formability

ABSTRACT

Provided is a 6000-series aluminum alloy sheet having good formability for an automotive body panel, which can be manufactured without greatly varying an existing composition or an existing manufacturing condition. The amount of solute Si and the amount of solute Cu in an Al—Mg—Si aluminum alloy sheet are increased in a balanced manner, so that the sheet has a dislocation density within a specific range when tensile deformation in a low-strain region is applied to the sheet. This suppresses localization of dislocations introduced into a material due to tensile deformation during press forming into an automotive body panel, and allows dislocations to be evenly multiplied from the low-strain region to a high-strain region. Consequently, uneven deformation is suppressed during forming into the automotive body panel, and good work hardenability is exhibited.

BACKGROUND

The present invention relates to an Al—Mg—Si aluminum alloy sheet having good formability. The aluminum alloy sheet described in the invention refers to a rolled sheet such as a hot-rolled sheet or a cold-rolled sheet, which has been subjected to tempering such as solution treatment and quenching, but has not been bent into a component to be used or has not been subjected to paint-bake hardening. Hereinafter, aluminum may be referred to as Al.

Recently, a social demand for weight saving of vehicles has increased more and more out of consideration for the global environment. To meet such a demand, a more lightweight aluminum alloy material having good formability and paint-bake hardenability (bake hardenability, hereinafter may be referred to as BH property) is increasingly used as a vehicle material in place of a steel material such as a steel sheet.

AA or JIS 6000-series Al—Mg—Si (hereinafter, may be simply referred to as 6000-series) aluminum alloy sheet is typically exemplified as an aluminum alloy sheet for a large automotive body panel such as an outer panel and an inner panel of an automobile. The 6000-series aluminum alloy sheet has a composition indispensably containing Si and Mg, which maintains formability due to its low proof stress (low strength) during forming, but is increased in proof stress (strength) through heating during artificial aging (hardening) such as paint-bake hardening of a formed panel so as to have a required strength, i.e., has good paint-bake hardenability.

In design, the automotive outer panel must achieve a beautiful curved-surface configuration and a beautiful character line without distortion or wrinkles even if a corner or a character line has a sharpened or complicated shape. The automotive inner panel must also achieve a beautiful curved-surface configuration without distortion or wrinkles even if a designed concavo-convex shape becomes deeper (higher) or complicated in relation to the outer panel.

Such a demand for high formability becomes strict every year along with expanded use of the aluminum alloy sheet as a material.

However, it is considerably difficult to achieve such good formability required for the automotive body panel application without greatly varying a typical (existing) alloy composition range, a typical manufacturing process, or a typical manufacturing condition of the 6000-series aluminum alloy sheet that is a material less workable than a steel sheet material.

In this regard, as well known, there have been suggested many approaches for controlling a composition or a microstructure to improve formability or strength characteristics of the material 6000-series aluminum alloy sheet for the body panel, the approaches including control of grain size, control of a texture, and control of atom cluster.

Such suggested approaches for microstructure control include various approaches such as control of the amount of solute Mg, control of the amount of solute Si, control of the amount of solute Cu, and control of dislocation density.

For example, Japanese Unexamined Patent Application Publication No. 2008-174797 suggests that the amount of solute Si is defined to be 0.55 to 0.80 mass %, the amount of solute Mg is defined to be 0.35 to 0.60 mass %, and a ratio of the amount of solute Si to the amount of solute Mg is defined to be 1.1 to 2 in order to produce a 6000-series aluminum alloy sheet that has good normal-temperature stability and is less likely to be deteriorated in material properties such as bake hardenability (BH property) through room temperature aging.

Japanese Unexamined Patent Application Publication No. 2008-266684 suggests a warm-forming 6000-series aluminum alloy sheet having good BH property, which has an amount of solute Cu of 0.01 to 0.7%, the amount being measured by a residue extraction method, and an average grain size of 10 to 50 μm.

T. Masuda; S. Hirosawa; Z. Horita; K. Matsuda Experimental and Computational Studies of Competitive Precipitation Behavior Observed in an Al—Mg—Si Alloy with High Dislocation Density and Ultrafine-Grained Microstructures, J. Japan Inst. Metals. 2011, 75(5), pp. 283-290 suggests that a microscopic structural parameter (dislocation density, grain size) as an optimum combination of dislocation strengthening or refining strengthening and precipitation strengthening is predicted to further increase strength of a 6000-series aluminum alloy sheet.

It is described that a specimen, which is prepared by performing cold rolling or HPT processing as one giant straining process on a 6000-series aluminum alloy sheet, is examined in dislocation density, and an unprocessed material has a dislocation density of about 10¹¹ m⁻², and a cold-rolled material subjected to a reduction of 30% (equivalent strain 0.36) has a dislocation density of about 10¹⁴ m⁻².

The dislocation density is measured by a cross analysis method using five view fields in a 100,000×TEM photograph with a fringe of equal thickness method.

SUMMARY

In such existing techniques, control of the amount of a solute element or control of dislocation density is performed to specifically improve strength characteristics of the 6000-series aluminum alloy sheet. Hence, although formability is naturally considered to be improved, such formability is still at a level of common hem bendability or press formability. That is, it is not intended to achieve severe and good formability as required for the recent automotive body panel.

Hence, the fact is that there have been only known measures to achieve such severe and good formability required for the automotive body panel application, such as a reduction in load during forming by modifying a panel design or a forming condition, or a reduction in strength during forming of the 6000-series aluminum alloy sheet.

An object of the invention, which has been given to solve such a problem, is to provide a 6000-series aluminum alloy sheet having good formability for the automotive body panel, the aluminum alloy sheet being manufactured without greatly varying a composition or manufacturing condition of the existing 6000-series aluminum alloy sheet.

To achieve the object, an aluminum alloy sheet having good formability of the invention is summarized by an Al—Mg—Si aluminum alloy sheet that contains, by mass percent, Si: 0.30 to 2.0%, Mg: 0.20 to 1.5%, Cu: 0.05 to 1.0%, Mn: 1.0% or less (not including 0%), and Fe: 1.0% or less (not including 0%), the remainder consisting of Al and inevitable impurities, in which the amount of solute Si is 0.30 to 2.0% and the amount of solute Cu is 0.05 to 1.0% in a solution of the aluminum alloy sheet, the solution being separated by a hot-phenol residue extraction method, and when tensile deformation with a strain of 5% is applied to the aluminum alloy sheet in a rolling direction of the aluminum alloy sheet, dislocation density in a rolled surface of the aluminum alloy sheet is 6.0×10¹⁴ to 12×10¹⁴ m⁻² in average, the dislocation density being measured by X-ray diffraction.

The invention is intended to increase the amount of solute Si and the amount of solute Cu in the 6000-series aluminum alloy sheet, and suppress localization of dislocations introduced into a material due to tensile deformation during forming into an automotive body panel, and thus uniformly (relatively highly) multiply dislocations from a low strain region to a high strain region of the tensile deformation.

This makes it possible to suppress uneven deformation from the high strain region to rupture in press forming into the automotive body panel, so that good work hardenability can be exhibited.

However, an important index is the amount of dislocation density in the sheet in the low strain region of tensile deformation to simulate actual forming into the automotive body panel in order to allow such a mechanism of solute Si or solute Cu to be securely exhibited, and securely achieve good formability for the automotive body panel.

In other words, it has been found that an increase in the amount of solute Si or solute Cu alone is not enough, and the amount of dislocation density in the sheet is also satisfied in the low strain region of tensile deformation, thereby good formability for the automotive body panel can be achieved.

It has been also found that the amount of dislocation density in the low strain region during forming (during tensile deformation) into an actual automotive body panel can be simulated by dislocation density at application of tensile deformation with a strain of 5% in a rolling direction of a material sheet, and thus the two dislocation densities correlate with each other.

Specifically, it is a necessary condition that the amount of solute Si and the amount of solute Cu in a material sheet are increased in a balanced manner, and it is a sufficient condition that when tensile deformation with a strain of 5% is applied to the material sheet in a rolling direction of the material sheet, the sheet has a predetermined dislocation density. Good formability for the automotive body panel can be achieved by satisfying such two conditions.

In addition, the good formability provided by such controls can be advantageously achieved without greatly varying an existing aluminum alloy composition or an existing manufacturing condition.

DETAILED DESCRIPTION

Hereinafter, an embodiment of the invention is specifically described for each of requirements.

Chemical Composition

A chemical composition of the Al—Mg—Si (hereinafter, may be referred to as 6000-series) aluminum alloy sheet of the invention is now described. The invention also satisfies the requirements for the properties required for the body panel in terms of the composition, the properties including good formability, a BH property, strength, weldability, and corrosion resistance. In such a case, however, it is also assumed that the existing composition and the existing manufacturing condition are not greatly varied.

To meet such a challenge in terms of the composition, the composition of the 6000-series aluminum alloy sheet contains, by mass percent, Si: 0.30 to 2.0%, Mg: 0.20 to 1.5%, Cu: 0.05 to 1.0%, Mn: 1.0% or less (not including 0%), and Fe: 1.0% or less (not including 0%), the remainder consisting of Al and inevitable impurities.

In addition, the composition may contain one or more of Cr: 0.3% or less (not including 0%), Zr: 0.3% or less (not including 0%), V: 0.3% or less (not including 0%), Ti: 0.1% or less (not including 0%), Zn: 1.0% or less (not including 0%), Ag: 0.2% or less (not including 0%), and Sn: 0.15% or less (not including 0%).

The content range, and the meaning, and/or the acceptable amount of each element of the 6000-series aluminum alloy sheet are now described. The percentage representing the content of each element refers to mass percent.

Si: 0.30 to 2.0%

Si is an indispensable element to provide strength (proof stress) required for the outer panel of a vehicle, which, with Mg, exhibits solid-solution strengthening, and forms Mg—Si precipitates that contribute to an increase in strength during artificial aging such as paint-bake treatment and thus exhibits artificial aging hardenability (BH property).

The solute Si suppresses localization of dislocations introduced into the material during press forming into the automotive body panel, and has an effect of evenly multiplying dislocations from the low strain region to the high strain region of tensile deformation. This makes it possible to suppress uneven deformation from the high strain region to rupture in press forming, so that large elongation and good work hardenability can be exhibited.

If the Si content is too small, the amount of solute Si decreases, leading to a reduction in elongation during press forming or deterioration in work hardenability. This results in a decrease in amount of dislocation multiplication after application of tensile deformation with a strain of 5%. In addition, since production of the Mg—Si precipitates becomes insufficient, the BH property is deteriorated, leading to a significant reduction in strength after paint-bake treatment.

If the Si content is too large, coarse particles and precipitates are formed, and a large crack occurs in the sheet during hot rolling.

Consequently, the Si content is within a range from 0.30 to 2.0%. The preferred lower limit of Si is 0.50%, and the preferred upper limit thereof is 1.5%.

Mg: 0.20 to 1.5%

Mg is also an indispensable element to provide proof stress required for the panel, which, with Si, exhibits solid-solution strengthening, and forms Mg—Si precipitates that contribute to an increase in strength during artificial aging such as paint-bake treatment and thus exhibits artificial aging hardenability (BH property).

As with the solute Si, the solute Mg suppresses localization of dislocations introduced into the material during press forming into the automotive body panel, and exhibits an effect of evenly multiplying dislocations from the low strain region to the high strain region of tensile deformation. This suppresses uneven deformation from the high strain region to rupture in press forming, allowing large elongation and good work hardenability to be exhibited.

If the Mg content is too small, the amount of solute Mg decreases, leading to deterioration in work hardenability. This results in a decrease in amount of dislocation multiplication after application of tensile deformation with a strain of 5%. In addition, since production of the Mg—Si precipitates becomes insufficient, the BH property is deteriorated, leading to a reduction in strength after paint-bake treatment.

If the Mg content is too large, coarse particles and precipitates are formed, and a large crack occurs in the sheet during hot rolling.

Consequently, the Mg content is within a range from 0.20 to 1.5%. The preferred lower limit of Mg is 0.30%, and the preferred upper limit thereof is 1.2%.

Cu: 0.05 to 1.0%

Cu contributes to an increase in strength and improvement in formability. As with the solute Si, solute Cu improves work hardenability, and improves a balance between strength and formability.

If the Cu content is less than 0.05%, the effect of Cu is small. In addition, the amount of solute Cu is also insufficient, and the effect of the solute Cu is also insufficient.

If the Cu content exceeds 1.0%, filiform corrosion resistance after painting and stress corrosion cracking resistance are significantly deteriorated. Hence, the Cu content is preferred to be 0.8% or less for an application in which corrosion resistance is important.

Mn: 1.0% or Less (not Including 0%)

Mn increases strength of an aluminum alloy through solid-solution strengthening and a grain refinement effect. However, if Mn is excessively contained to exceed 1.0%, the amount of Al—Mn intermetallic compounds increases and fracture origins are caused, and thus elongation easily decreases. When a low strain of about 5% is applied to the sheet, dislocations are localized around the Al—Mn intermetallic compounds, and thus work hardenability is also deteriorated.

Consequently, the Mn content is 1.0% or less (not including 0%), preferably 0.80% or less (not including 0%).

Fe: 1.0% or Less (not Including 0%)

Fe forms Al—Fe intermetallic compounds in an aluminum alloy. Hence, if the Fe content increases, the amount of such compounds increases and fracture origins are caused, and thus elongation easily decreases. In addition, each Al—Fe intermetallic compound often includes Si, and thus the amount of solute Si is decreased in correspondence to the amount of Si captured by the intermetallic compound.

Fe is contaminated into an aluminum alloy as a bullion impurity, and the content of Fe increases with an increase in amount of aluminum alloy scrap (ratio relative to aluminum bullion); hence, the smaller the Fe content, the better. However, decreasing the Fe content to the detection limit or lower leads to an increase in cost; hence, a certain level of Fe content must be allowed.

Consequently, the Fe content is 1.0% or less (not including 0%), preferably 0.5% or less (not including 0%).

Other Elements

In addition, the invention allows the composition to further contain one or more of Cr: 0.3% or less (not including 0%), Zr: 0.3% or less (not including 0%), V: 0.3% or less (not including 0%), Ti: 0.1% or less (not including 0%), Zn: 1.0% or less (not including 0%), Ag: 0.2% or less (not including 0%), and Sn: 0.15% or less (not including 0%).

Such elements in common exhibit an effect of increasing strength of a sheet, and thus can be considered to be equieffective in increasing strength. However, specific mechanisms of the effect are in common on the one hand, but are naturally different on the other hand.

As with Mn, each of Cr, Zr, and V forms dispersed particles (dispersed phase) during homogenization. Such dispersed particles have an effect of preventing grain boundary migration after recrystallization, and refine grains.

Ti, with B, forms particles, which serve as nuclei of recrystallized grains, and thus prevents coarsening of grains, and refines the grains.

Each of Zn and Ag is useful for improving artificial aging performance (BH property), and exhibits an effect of promoting precipitation of a compound phase such as a GP zone into a grain boundary of a sheet microstructure under a condition of relatively low temperature and relatively short artificial aging.

Sn captures atomic vacancies and thus suppresses diffusion of Mg or Si at room temperature, and thus suppresses an increase in strength at room temperature (room temperature aging), and releases the captured vacancies and thus promotes diffusion of Mg or Si, leading to an effect of improving the BH property.

However, if the content of each of such elements is too large, coarse compounds are formed, making it difficult to manufacture the sheet. Furthermore, this reduces strength, formability such as bendability, and corrosion resistance. Consequently, when each of such elements is contained, the content of the element is equal to or lower than the upper limit.

Microstructure

Assuming such an alloy composition, the invention also defines a microstructure of the sheet in order to improve formability, the microstructure including the amount of solute Si, the amount of solute Cu, and dislocation density as described below.

Amount of Solute Si and Amount of Solute Cu

For automotive body panel application, the amount of solute Si and the amount of solute Cu in the 6000-series aluminum alloy sheet have been mainly controlled to improve strength characteristics as described in Japanese Unexamined Patent Application Publication Nos. 2008-174797 and 2008-266684.

On the other hand, in the invention, formability into the automotive body panel is improved by increasing the amount of solute Si and the amount of solute Cu in a balanced manner.

The inventors have not found the case where the amount of solute Si and the amount of solute Cu in the 6000-series aluminum alloy sheet for the automotive body panel application are controlled to improve formability.

A defined range of each of the amount of solute Si and the amount of solute Cu and the meaning of the range are now described.

Amount of Solute Si 0.30 to 2.0%

A larger amount of solute Si reduces stacking-fault energy of aluminum alloy in conjunction with solute Cu, and suppresses localization of dislocations introduced into a material during tensile deformation, e.g., during press forming into an automotive body panel, and thus evenly multiplies dislocations from the low strain region to the high strain region of tensile deformation. As a result, work hardenability is improved, a yield ratio is decreased, and elongation increases.

If the amount of solute Si is less than 0.30%, the effect is insufficient even if the amount of solute Cu is satisfied.

The upper limit of the amount of solute Si is substantially equal to the upper limit of the Si content.

As with the solute Si, solute Mg also improves work hardenability, reduces a yield ratio, and increases elongation.

However, while control of the amount of solute Si is complicated and important because Si is precipitated together with Al—Fe or Al—Mn intermetallic compounds, the amount of solute Mg is relatively easily controlled because Mg is mainly precipitated with Si.

Furthermore, since fluctuation of the amount of solute Mg shows the same behavior or tendency as that of the amount of solute Si, if only the amount of solute Si is measured and controlled to satisfy the definition, the amount of solute Mg necessarily falls within a preferred range, and thus the amount of solute Mg is not necessary to be measured and controlled.

In the invention, therefore, the amount of solute Mg is not particularly defined while functions and effects of the solute Mg are expected.

Amount of Solute Cu 0.05 to 1.0%

The amount of solute Cu is also important in addition to the amount of solute Si. As with solute Si, a larger amount of solute Cu also improves work hardenability, decreases a yield ratio, and increases elongation, and thus improves a balance between strength and formability.

If the amount of solute Cu is less than 0.05%, the effect is insufficient even if the amount of solute Si is satisfied.

The upper limit of the amount of solute Cu is substantially equal to the upper limit of the amount of added Cu.

Dislocation Density

To allow the above-described mechanism of solute Si or solute Cu to be securely exhibited to securely achieve good formability for the automotive body panel, it is necessary not only to control the amount of solute Si and the amount of solute Cu, but also to control the amount of dislocation density in the sheet in the low strain region during forming into the actual automotive body panel.

Such an amount of dislocation density in the low strain region can be reproducibly measured by dislocation density in a sheet to which tensile deformation with a strain of 5% is applied in a rolling direction of the sheet to simulate press forming into an actual automotive body panel.

Hence, in the invention, a sheet, which satisfies the composition, the amount of solute Si, and the amount of solute Cu, is subjected to a tensile test simulating the press forming into an actual automotive body panel. Dislocation density of the sheet, to which the tensile deformation with a strain of 5% is (has been) applied, is controlled into a range from 6.0×10¹⁴ to 12×10¹⁴ m⁻².

The dislocation density is determined through measurement by X-ray diffraction of a microstructure of a rolled surface (rolled plane) of the sheet to which tensile deformation with a strain of 5% has been applied in a rolling direction of the sheet.

Dislocations are multiplied evenly (relatively highly) in the above-described range in the low strain region in which a strain of about 5% is shown at the tensile test. Uneven deformation is thus suppressed from the subsequent high strain region to rupture, and good work hardenability (a decrease in yield ratio, an increase in elongation) is exhibited.

The dislocation density of lower than 6.0×10¹⁴ m⁻² suggests that dislocations are less likely to be multiplied, i.e., work hardenability is bad. This causes early rupture in the high strain region, leading to deterioration of formability.

Conversely, the dislocation density of higher than 12×10¹⁴ m⁻² decreases dislocations that can be introduced and accumulated in the subsequent high strain region; hence, formability is also not improved.

Consequently, when (after) tensile deformation of 5% is applied in a rolling direction of the sheet, dislocation density is defined to be within a range from 6.0×10¹⁴ to 12×10¹⁴ m⁻², preferably 7.0×10¹⁴ to 11×10¹⁴ m⁻² in average.

As described in J. Japan Inst. Metals. 2011, 75(5), pp. 283-290, a typical 6000-series aluminum alloy sheet, to which tensile deformation with a strain of 5% is not applied unlike in the invention, has a dislocation density of only about 10¹¹ m⁻² in an unprocessed state (solution-treated material) while simple comparison is difficult because such a value is measured by a different method (measured with TEM with 100,000 magnifications). The sheet has a dislocation density of only about 10¹⁴ m⁻² while being subjected to cold rolling with a reduction of 30% (equivalent strain 0.36).

In contrast, in the invention, tensile deformation with a low strain of only 5% is merely applied to a solution-treated cold-rolled sheet, thereby the dislocation density of 6.0×10¹⁴ to 12×10¹⁴ m⁻² can be introduced, the dislocation density exceeding dislocation density applied by cold rolling in J. Japan Inst. Metals. 2011, 75(5), pp. 283-290.

This is because of the increase in each of the amount of solute Si and the amount of solute Cu in the invention. This means that the dislocation density defined by the invention cannot be introduced without such an increase. Furthermore, this means that a mechanism of strain or dislocation density introduced into a material is completely different between tensile deformation during forming into an automotive body panel and cold rolling of a sheet as in J. Japan Inst. Metals. 2011, 75(5), pp. 283-290.

The technical idea of the invention, i.e., the idea of increasing the amount of solute Si and the amount of solute Cu is given only after recognizing the relationship between formability into the automotive body panel and the amount of solute Si or the amount of solute Cu.

The technical idea of controlling the amount of dislocation density in a sheet is also given only after noting dislocation density introduced into a material due to tensile deformation, particularly dislocation density in the low strain region during forming into an automotive body panel, for example.

Furthermore, the understanding that a dislocation density as high as 6.0×10¹⁴ to 12×10¹⁴ m⁻² can be introduced only by applying tensile deformation with a low strain of only 5% to a solution-treated material (unprocessed material) is given only after getting the technical idea and confirming the idea through an actual test.

Hence, even if known examples including J. Japan Inst. Metals. 2011, 75(5), pp. 283-290 have noted influence of dislocation density in a sheet on properties such as strength of the sheet, or even if there is a known example where sheet strength is increased by increasing the amount of solute Si or the amount of solute Cu, the configuration of the invention is not easily obtained.

Measurement Method of Dislocation Density

Dislocation density is generally measured by, for example, a transmission electron microscope as in J. Japan Inst. Metals. 2011, 75(5), pp. 283-290. In the invention, dislocation density is measured more simply and reproducibly by X-ray diffraction.

A region dense with linear or streaky dislocations (a cell wall or shear band) in a dislocation is difficult to be determined by the transmission electron microscope, and may cause measurement error for obtaining dislocation density ρ. On the other hand, X-ray diffraction is advantageous in that errors are decreased even for such a forest dislocation because the dislocation density ρ is calculated from half value widths of diffraction peaks from various faces of a texture as described later.

In a microstructure of a sheet into which dislocations are introduced by applying plastic deformation through cold rolling or a tensile test, lattice distortion occurs around a dislocation. In addition, a low-angle grain boundary or a cell structure is developed with dislocation arrangement. When such a dislocation and a domain structure associated with the dislocation are taken from an X-ray diffraction pattern, a distinctive spread or shape corresponding to a diffraction index appears in a diffraction peak. Dislocation density can be determined through analysis (line profile analysis) of the diffraction peak shape (line profile).

Specifically, first, a JIS Z2201 No. 5 test specimen (25 mm×50 mm gage length (GL)×thickness) is taken as a test sample sheet from a tempered cold-rolled sheet according to a procedure of a tensile test, and the test specimen is stretched at room temperature in a rolling direction as a tensile direction. This is to simulate a dislocation density of a sheet in a low strain region during forming into an actual automotive body panel, and tensile deformation with a strain of 5% is applied as the low strain region.

A microstructure of a rolled surface (rolled plane) of the test specimen, to which tensile deformation with a strain of 5% is applied, is subjected to X-ray diffraction to obtain half value widths of diffraction peaks from the faces (bearing faces) of (111), (200), (220), (311), (400), (331), (420), and (422) as major orientations of a texture of a surficial portion of the sheet (test specimen). The half value width of the diffraction peak of each face increases with an increase in dislocation density ρ. The rolled surface to be measured by X-ray diffraction of the test specimen, to which tensile deformation with a strain of 5% is applied, may be left as it is, or may be washed without etching.

Subsequently, lattice distortion (crystal distortion) ε is obtained from the half value widths of the diffraction peaks from the faces by the Williamson-Hall method, and then the dislocation density ρ can be calculated by the following formula.

ρ=16.1ε² /b ²

where ρ is dislocation density, e is lattice distortion, and b is magnitude of Burgers vector.

Moreover, 2.8635×10⁻¹⁰ m is used as the magnitude of Burgers vector.

The Williamson-Hall method is a known line profile analysis that is generally used to determine dislocation density or grain size from a relationship between a plurality of half value widths of diffraction and a plurality of diffraction angles. Such a series of ways to determine dislocation density by X-ray diffraction are also well known. The invention generally refers to the series of ways to determine dislocation density by X-ray diffraction as “dislocation density measured by X-ray diffraction”.

Index of Good Work Hardenability (Good Formability)

An index (guideline) of achievement of good work hardenability (good formability) by the control of the composition and the microstructure includes yield ratio and elongation.

A low yield ratio and large elongation support better formability for the automotive body panel without a forming test of a sheet with a small test specimen or without a forming test of a sheet into an actual automotive body panel.

Specifically, the index (guideline) of achievement of good formability is that a yield ratio, which is defined by a ratio of 0.2% proof stress to tensile strength (0.2% proof stress/tensile strength), of an aluminum alloy sheet is 0.56 or less, and total elongation is 26% or more as supported by Example described later.

If the yield ratio is excessively high to extend 0.56, or if the total elongation is excessively small, less than 26%, the good work hardenability or good formability for the automotive body panel cannot be achieved.

Manufacturing Method

A method of manufacturing the aluminum alloy sheet of the invention is now described.

The aluminum alloy sheet of the invention is manufactured by a common or known manufacturing process, in which an aluminum alloy slab having the 6000-series composition is casted, and is then subjected to homogenization, hot rolling, and cold rolling in order and thus formed into a sheet having a predetermined thickness, and then the sheet is subjected to tempering such as solution hardening.

In such a manufacturing process, however, as described later, a soaking condition, a hot finish rolling condition, and a solution condition, and a quenching condition are each adjusted to be within a preferred range in order to securely and reproducibly provide the microstructure (the amount of solute Si and the amount of solute Cu, or dislocation density) defined by the invention.

Cooling Rate in Melting and Casting

In a melting-and-casting step, molten metal of aluminum alloy, which is melted and adjusted to be within the 6000-series composition range, is casted by an appropriately selected common melting-and-casting process such as a continuous casting process and a semi-continuous casting process (DC casting process). The average cooling rate during casting is preferably controlled to be as high (fast) as possible, i.e., 30° C./min or more from the liquidus temperature to the solidus temperature in order to control the microstructure (the amount of solute Si and the amount of solute Cu, or dislocation density) within the range defined by the invention.

If such temperature (cooling rate) control in a high temperature region during casting is not performed, the cooling rate in the high temperature region inevitably becomes lower. If the cooling rate in the high temperature region thus becomes lower, an increased amount of coarse particles are produced within the temperature range of the high temperature region, leading to a decrease in amount of solute Si and in amount of solute Cu in the slab. As a result, it is difficult to control the microstructure to be within the range of the invention.

Homogenization

Subsequently, the casted aluminum alloy slab is subjected to homogenization prior to hot rolling. The homogenization (soaking) is important for sufficient solid solution of Si and Mg in addition to homogenization of a microstructure (eliminating segregation in a grain of a slab microstructure) as a common purpose. Any homogenization condition including common onetime or one-stage treatment may be used without limitation as long as such purposes are achieved.

Homogenization temperature is 500 to 560° C., and homogenization (holding) time is appropriately selected from a range of 1 hr or more to sufficiently dissolve Si and Cu. If the homogenization temperature is low, the amount of solute Si or Cu cannot be provided, and the microstructure (the amount of solute Si and the amount of solute Cu) defined by the invention cannot be produced even by pre-aging (reheating) after solution hardening as described later. In addition, segregation in the grain cannot be sufficiently eliminated, which serves as an origin of fracture, leading to deterioration in formability.

After the homogenization, the slab is hot-rolled, in which the temperature of the slab is not decreased to 500° C. or lower before start of hot rough rolling after the homogenization in order to provide the amount of solute Si and the amount of solute Cu.

If temperature of the slab is decreased to 500° C. or lower before start of rough rolling, Si or Cu is precipitated. It is therefore more difficult to provide a certain amount of solute Si or solute Cu to form the microstructure defined by the invention.

Hot Rolling

Hot rolling includes a rough rolling step for the slab and a finish rolling step depending on thickness of the sheet to be rolled. A reverse-type or tandem-type rolling mill is appropriately used for the rough rolling step or the finish rolling step.

During rolling from start to finish of hot rough rolling, it is necessary to maintain the amount of solute Si and the amount of solute Mg without lowering temperature to 450° C. or lower.

If minimum interpass temperature of a rough rolled sheet is lowered to 450° C. or lower due to, for example, increased rolling time, Mg—Si compounds are easily precipitated, and thus the amount of solute Si and the amount of solute Cu are decreased. It is therefore more difficult to provide a certain amount of solute Si or solute Cu to form the microstructure defined by the invention.

After such hot rough rolling, the slab is subjected to hot finish rolling with finish temperature in a range from 300 to 360° C.

If finish temperature of the hot finish rolling is extremely low, lower than 300° C., a rolling load increases and productivity is reduced. On the other hand, if the finish temperature of the hot finish rolling is increased to form a recrystallized structure while a large amount of worked structure is not left, the finish temperature of more than 360° C. causes the Mg—Si compounds to be easily precipitated, leading to a decrease in each of the amount of solute Si and the amount of solute Cu. It is therefore difficult to provide the amount of solute Si or solute Cu to form the microstructure defined by the invention.

An average cooling rate from the material (sheet) temperature immediately after finish of the hot finish rolling to the material temperature of 150° C. is controlled to at least 5° C./hr.

If the average cooling rate is lower than 5° C./hr, a large amount of Mg—Si precipitates are produced during such cooling, and the amount of solute Si in a product sheet is decreased.

Hence, the average cooling rate immediately after finish of the hot finish rolling is preferably higher, and is at least 5° C./hr or higher, preferably 8° C./hr or higher.

Annealing of Hot-Rolled Sheet

Although annealing (heat treatment) of the hot-rolled sheet before cold rolling is not necessary, the annealing may be performed.

Cold Rolling

In cold rolling, the hot-rolled sheet is rolled and formed into a cold-rolled sheet (including a coil) having a desired final thickness. Cold reduction is desirably 30% or more to further refine the grains. In addition, intermediate annealing may be performed between cold rolling passes for the same purpose as that of the heat treatment.

Solution Treatment and Quenching

The cold-rolled sheet is subjected to solution treatment and subsequent quenching to room temperature. The solution hardening may be performed using a typical continuous heat treatment line.

However, to provide a sufficient solid-solution amount of each element such as Mg and Si, the cold-rolled sheet is preferably heated to a solution treatment temperature of 550° C. or higher and equal to or lower than the melting point and held at the temperature for 10 sec or more, and then cooled with a preferred average cooling rate of 20° C./sec or more from such holding temperature to 100° C.

If the temperature is lower than 550° C., or if the holding time is shorter than 10 see, reversion of Cu-containing Al—Mn, Al—Fe, or Mg—Si compounds, which have been produced before the solution treatment, is insufficient, and the amount of solute Si and the amount of solute Cu are decreased.

If the average cooling rate is less than 20° C./sec, Mg—Si precipitates are mainly produced during cooling and thus the amount of solute Si is decreased. Consequently, the amount of solute Si is also difficult to be provided. To achieve such a cooling rate, cooling methods such as air cooling with a fan and water cooling with mist, spray, or dipping, and conditions are selectively used for the quenching.

Pre-Aging: Reheating

Pre-aging is performed after such solution treatment and quenching as necessary. The pre-aging has a small influence on the amount of solute Si or the amount of solute Cu, and is selectively performed if improvement in BH property is necessary, for example.

The pre-aging (reheating), if performed, is preferably performed within one hour after the sheet is subjected to the quenching and cooled to room temperature.

If room-temperature holding time from finish of the room-temperature quenching to start of pre-aging (start of reheating) is too long, an Mg—Si cluster that does not contribute to the BH property is formed due to room-temperature aging, and an Mg—Si cluster having a good balance of Mg and Si, which contributes to the BH property, is less likely to be increased. Hence, the shorter the room-temperature holding time, the better. That is, the solution treatment and quenching may be followed by the reheating with substantially no time difference, and lower-limit time is not specifically set.

In the pre-aging, holding time ranging from 60 to 120° C. is 10 to 40 hr. This results in formation of the Mg—Si cluster having a good balance of Mg and Si.

Although the invention is now described in detail with Example, the invention should not be limited thereto, and modifications or alterations thereof may be made within the scope without departing from the gist described before and later, all of which are included in the technical scope of the invention.

Example

Example of the invention is now described. 6000-Series aluminum alloy sheets, which had different compositions as shown in Table 1 and different microstructures as shown in Table 2, each microstructure including the amount of solute Si, the amount of solute Cu, and dislocation density after application of tensile deformation of 5%, were appropriately manufactured under different manufacturing conditions.

Each of the manufactured sheets was held for 10 days at room temperature (subjected to room-temperature aging), and then the amount of solute Si, the amount of solute Cu, dislocation density after application of tensile deformation of 5%, 0.2% proof stress, tensile strength, a yield ratio (0.2% proof stress/tensile strength), and total elongation were measured and evaluated. Table 2 also shows results of those. Table 2 is the rest of Table 1, and respective alloy numbers in Table 1 correspond to alloy numbers in Table 2.

In the specific appropriate manufacturing method, the 6000-series aluminum alloy sheets having the chemical compositions as shown in Table 1 were manufactured under different manufacturing conditions as shown in Table 2, each manufacturing condition including soaking temperature, minimum interpass temperature of a rough rolled sheet in hot rough rolling (shown as minimum temperature in Table 2), finish temperature of hot finish rolling, average cooling rate from material (sheet) temperature immediately after finish of hot finish rolling to material temperature of 150° C., and holding temperature and average cooling rate of solution treatment.

In representation of the content of each element in Table 1, representation with no numerical value for each element indicates that the content of the element is equal to or lower than the detection limit.

Specific manufacturing conditions of the aluminum alloy sheets were as follows. Aluminum alloy slabs having the compositions shown in Table 1 were in common melted by a DC casting process. This melting was in common performed such that the average cooling rate during the casting was 50° C./min from the liquidus temperature to the solidus temperature. Subsequently, the slabs were in common soaked for six hours, and were then subjected to hot rough rolling at the temperature. Table 2 also shows the minimum (pass) temperature of that hot rough rolling.

The slabs were in common subjected to subsequent hot finish rolling such that the slabs were hot-rolled into a thickness of 2.5 mm with finish temperatures and the average cooling rates (° C./hr) after finish of the rolling as shown in Table 2, and thus the slabs were formed into hot-rolled sheets.

The hot-rolled aluminum alloy sheets were in common subjected to heat treatment of 500° C.×1 min, and was then cold-rolled with a reduction of 50% without process annealing between cold rolling passes, and were thus formed into cold-rolled sheets 1.0 mm in thickness.

Furthermore, the cold-rolled sheets were in common continuously subjected to tempering (T4) while being rewound and wound up in continuous heat treatment equipment. Specifically, the solution treatment was performed in such a manner that each of the cold-rolled sheets was heated to the target temperature (holding temperature) listed in Table 2 with an average heating rate of 50° C./sec below 500° C., and then the cold-rolled sheets were in common held at the target temperature for 20 sec and then water-cooled to room temperature at the average cooling rates (° C./sec) listed in Table 2.

Test sample sheets (blanks) were cut from each final product sheet that was left at room temperature for 10 days after such tempering, and the amount of solute Si and the amount of solute Cu, a microstructure defined by dislocation density, and mechanical properties of each test sample sheet were measured and evaluated. Table 2 shows results of those.

Measurement of Amount of Solute Si and Amount of Solute Cu

The amount of solute Si and the amount of solute Cu in each of the test sample sheets were measured by the hot-phenol residue extraction method as follows: A sample to be measured was dissolved, solid and liquid were separated and classified through filtration separation with a filter having a mesh of 0.1 μm, and the content of Si and the content of Cu in the separated solution were measured as the amount of solute Si and the amount of solute Cu, respectively.

The hot-phenol residue extraction method was specifically performed as follows. First, phenol was put into a decomposition flask and heated, and then each test sample sheet to be measured was transferred into the decomposition flask and thermally decomposed. Subsequently, benzyl alcohol was added, and solid and liquid were separated and classified by suction filtration with the filter, and the content of Si and the content of Cu in the separated solution were each quantitatively analyzed.

The atomic absorption analysis (AAS) or the inductively-coupled plasma emission spectrometry (ICP-OES) was appropriately used for the quantitative analysis.

A 47 mm diameter membrane filter having a mesh (collection particle size) of 0.1 μm as described above was used for the suction filtration.

Such measurement and calculation were performed for each of three samples taken at three points in total including one point in the center in a sheet width direction and two points at both ends in the sheet width direction from the center of the test sample sheet, and the amounts (mass %) of each of solute Si and solute Cu in the samples were averaged and defined as the amount of each of solute Si and solute Cu in the sheet.

Measurement of Dislocation Density

Tensile deformation with a strain of 5% was applied to each of the test sample sheets (sampled test specimens) in a rolling direction according to the above-described procedure, and dislocation density (×10¹⁴ m⁻²) in a rolled surface was measured by X-ray diffraction under the above-described specific condition. The measurement was performed for each of appropriate five points on each test sample sheet, and an average of the dislocation densities at the five points was defined as average dislocation density (×10¹⁴ m⁻²)

Tensile Test

The tensile test of each test sample sheet was performed at room temperature with a JIS Z2201 No. 5 test specimen (25 mm×50 mm gage length (GL)×thickness) taken from each test sample sheet. The tensile direction of the test specimen was a direction parallel to the rolling direction. The tensile speed was 5 mm/min below the 0.2% proof stress, and was 20 mm/min at or above the 0.2% proof stress. The number N of times of measurement of each of the mechanical properties was three, and an average of the measured values was calculated for each property.

For each test sample sheet, 0.2% proof stress, tensile strength, a yield ratio (0.2% proof stress/tensile strength), and total elongation were calculated.

As shown in Tables 1 and 2, inventive examples 1 to 11 are each within a range of the chemical composition of the invention, and are each manufactured within the range of the preferred condition.

In the inventive examples, therefore, as shown in Table 2, the amount of solute Si is 0.30 to 2.0% and the amount of solute Cu is 0.05 to 1.0% in the solution separated by the hot-phenol residue extraction method, and when tensile deformation with a strain of 5% is applied to the sheet in a rolling direction of the sheet, dislocation density in a rolled surface of the sheet is 6.0×10¹⁴ to 12×10¹⁴ m⁻² in average, the dislocation density being measured by X-ray diffraction, as defined by the invention.

As a result, as shown in Table 2, each inventive example exhibits a yield ratio of 0.56 or less, the yield ratio being defined by a ratio of 0.2% proof stress to tensile strength (0.2% proof stress/tensile strength), and total elongation of 26% or more even after room-temperature aging, i.e., has a good formability acceptable for the automotive body panel.

On the other hand, although comparative examples 12 to 16 in Table 2 are each manufactured within a preferred condition range, they use the alloy Nos. 12 to 16, respectively, in each of which the content of at least one of Si, Mg, Cu, Mn, and Fe is out of the range of the invention.

Hence, as shown in Table 2, such comparative examples are each bad in formability compared with the inventive examples, in which one of the amount of solute Si, the amount of solute Cu, and average dislocation density in the low strain region is out of the range defined by the invention, and furthermore the yield ratio exceeds 0.56, or total elongation is less than 26%. Consequently, the comparative examples are unacceptable for the automotive body panel.

The comparative example No. 12 corresponds to alloy 12 in Table 1, in which the content of Mg is excessively small.

The comparative example No. 13 corresponds to alloy 13 in Table 1, in which the content of Si is excessively small.

The comparative example No. 14 corresponds to alloy 14 in Table 1, in which the content of Cu is excessively small.

The comparative example No. 15 corresponds to alloy 15 in Table 1, in which the content of Mn is excessively large.

The comparative example No. 16 corresponds to alloy 16 in Table 1, in which the content of Fe is excessively large.

Comparative examples 17 to 21 in Table 2 each use the alloy within the range of the invention as shown in Table 1. However, as shown in Table 2, such comparative examples are each out of a preferred manufacturing condition including soaking temperature, minimum temperature of hot rough rolling, finish temperature of hot finish rolling, average cooling rate (° C./hr) after finish of the hot finish rolling, and holding temperature and average cooling rate (° C./sec) of solution treatment.

As a result, at least one of the amount of solute Si, the amount of solute Cu, and average dislocation density in the low strain region is out of the range defined by the invention, the yield ratio exceeds 0.56, or total elongation is less than 26%, unlike the inventive examples. Consequently, the comparative examples are unacceptable for the automotive body panel.

In the comparative example 17, the soaking temperature and the minimum temperature of hot rough rolling are each excessively low. Hence, the amount of solute Si and the amount of solute Cu are each excessively small to be below the lower limit, and the average dislocation density in the low strain region is also excessively low. Consequently, the yield ratio exceeds 0.56 and the total elongation is less than 26%, leading to bad formability.

In the comparative example 18, the minimum temperature of hot rough rolling and the finish temperature of hot finish rolling are each excessively low. Hence, the amount of solute Si and the amount of solute Cu are each excessively small to be below the lower limit, and the average dislocation density in the low strain region is also excessively low. Consequently, the yield ratio exceeds 0.56 and the total elongation is less than 26%, leading to bad formability.

In the comparative example 19, the average cooling rate (° C./hr) after finish of the hot finish rolling is excessively low. Hence, the amount of solute Si is excessively small to be below the lower limit, and the average dislocation density in the low strain region is also excessively low. Consequently, the yield ratio exceeds 0.56 and the total elongation is less than 26%, leading to bad formability.

In the comparative example 20, the holding temperature of solution treatment is excessively low. Hence, the amount of solute Si and the amount of solute Cu are each excessively small to be below the lower limit, and the average dislocation density in the low strain region is also excessively low. Consequently, the yield ratio exceeds 0.56 and the total elongation is less than 26%, leading to bad formability.

In the comparative example 21, average cooling rate (° C./sec) after solution treatment is excessively low. Hence, the amount of solute Si is excessively small to be below the lower limit, and the average dislocation density in the low strain region is also excessively low. Consequently, although the amount of solute Cu satisfies the definition, the yield ratio exceeds 0.56 and the total elongation is less than 26%, leading to bad formability.

These results of the Example support the meaning of satisfying all the requirements of the composition and the microstructure defined by the invention, the requirements being to produce a 6000-series aluminum alloy sheet having good formability for the automotive body panel without greatly varying the existing composition or manufacturing condition.

TABLE 1 Chemical composition of aluminum alloy sheet (mass %, the remainder: Al) No. Si Mg Cu Mn Fe Cr Zr V Ti Zn Ag Sn 1 1.0 0.45 0.20 0.07 0.17 2 0.50 0.65 0.08 0.08 0.18 3 1.5 0.43 0.19 0.07 0.17 0.05 4 0.96 0.47 0.20 0.08 0.15 0.10 5 0.98 0.30 0.24 0.07 0.16 0.10 0.03 0.05 6 1.1 0.45 0.18 0.07 0.20 0.12 7 0.83 0.25 0.41 0.19 0.10 0.20 8 0.36 0.95 0.80 0.50 0.08 0.20 0.15 9 1.7 1.4 0.06 0.05 0.28 0.20 10 0.95 0.63 0.25 0.78 0.11 0.07 11 1.0 1.2 0.90 0.32 0.45 0.70 0.05 12 0.67 0.16 0.18 0.08 0.15 0.05 0.30 0.03 13 0.27 1.0 0.44 0.81 0.33 0.08 0.05 14 1.1 0.72 0.02 0.28 0.21 0.10 15 1.3 0.80 0.13 1.3 0.17 0.10 0.05 16 0.52 0.75 0.21 0.11 1.2 0.10 0.05 0.10 17 0.60 0.65 0.13 0.08 0.18 18 0.60 0.65 0.13 0.08 0.18 19 0.60 0.65 0.13 0.08 0.18 20 0.60 0.65 0.13 0.08 0.18 21 0.60 0.65 0.13 0.08 0.18

TABLE 2 Manufactoring condition of aluminum alloy sheet Hot finish rolling Average cooling rate Aluminum alloy sheet held for 10 days at room temperature (° C./hr) Average from dislocation Hot material density Property rough temperature Solution (×10¹⁴ m⁻²) Yield Soaking rolling immediately treatment after ratio Soaking Minimum Finish after Holding Average Solute tensile 0.2% (proof temper- temper- temper- finish of temper- cooling amount deformation Proof Tensile stress/ Elon- ature ature ature rolling to ature rate (mass %) of 5% strength strength tensile gation Classification No. (° C.) (° C.) (° C.) 150° C. (° C.) (° C./sec) Si Cu (×10¹⁴ m⁻²) (MPa) (MPa) strength (%) Inventive 1 540 470 330 10 570 30 0.85 0.19 9.0 135 250 0.54 31 example 2 550 480 340 6 560 25 0.46 0.07 7.2 125 240 0.52 27 3 500 480 330 10 570 25 0.82 0.14 8.8 128 232 0.55 29 4 540 450 310 10 560 30 0.83 0.18 8.9 133 242 0.55 30 5 540 470 360 20 560 25 0.84 0.22 9.0 138 249 0.55 30 6 530 460 320 10 550 20 0.87 0.16 9.3 117 240 0.49 32 7 560 480 350 15 570 30 0.73 0.36 8.2 124 244 0.51 31 8 530 470 320 20 560 25 0.31 0.69 6.3 130 256 0.51 28 9 510 460 310 10 550 30 1.1 0.07 11 145 265 0.55 29 10 520 470 330 15 560 25 0.78 0.22 8.5 137 253 0.54 31 11 520 460 320 10 550 40 0.75 0.78 8.3 139 264 0.53 30 Comparative 12 540 470 330 10 570 30 0.51 0.15 5.8 85 150 0.57 26 example 13 540 470 330 10 570 30 0.26 0.35 5.5 119 218 0.55 24 14 540 470 330 10 570 30 0.82 0.02 8.6 145 256 0.57 27 15 540 470 330 10 570 30 1.0 0.09 13 150 260 0.58 24 16 540 470 330 10 570 30 0.25 0.18 5.5 91 166 0.55 24 17 480 420 300 10 560 30 0.27 0.03 5.7 118 205 0.58 23 18 500 430 280 10 560 30 0.28 0.04 5.8 120 211 0.57 23 19 530 470 320 3 560 25 0.28 0.08 5.8 124 217 0.57 24 20 540 470 330 10 530 30 0.25 0.03 5.5 119 205 0.58 23 21 540 470 330 10 570 15 0.27 0.06 5.6 122 215 0.57 24

According to the invention, a 6000-series aluminum alloy sheet having good formability for the automotive body panel can be produced without greatly varying the existing composition or manufacturing condition. As a result, use of the 6000-series aluminum alloy sheet for the automotive body panel can be expanded. 

What is claimed is:
 1. An aluminum alloy sheet having good formability, comprising an Al—Mg—Si aluminum alloy sheet that contains, by mass percent, Si: 0.30 to 2.0%, Mg: 0.20 to 1.5%, Cu: 0.05 to 1.0%, Mn: 1.0% or less (not including 0%), and Fe: 1.0% or less (not including 0%), the remainder consisting of Al and inevitable impurities, wherein an amount of solute Si is 0.30 to 2.0% and an amount of solute Cu is 0.05 to 1.0% in a solution of the aluminum alloy sheet, the solution being separated by a hot-phenol residue extraction method, and when tensile deformation with a strain of 5% is applied to the aluminum alloy sheet in a rolling direction of the aluminum alloy sheet, dislocation density in a rolled surface of the aluminum alloy sheet is 6.0×10¹⁴ to 12×10¹⁴ m⁻² in average, the dislocation density being measured by X-ray diffraction.
 2. The aluminum alloy sheet according to claim 1, wherein the aluminum alloy sheet further contains one or more of Cr: 0.3% or less (not including 0%), Zr: 0.3% or less (not including 0%), V: 0.3% or less (not including 0%), Ti: 0.1% or less (not including 0%), Zn: 1.0% or less (not including 0%), Ag: 0.2% or less (not including 0%), and Sn: 0.15% or less (not including 0%).
 3. The aluminum alloy sheet according to claim 1, wherein a yield ratio is 0.56 or less, the yield ratio being defined by a ratio of 0.2% proof stress to tensile strength (0.2% proof stress/tensile strength) of the aluminum alloy sheet, and a total elongation is 26% more. 